RF communications systems include RF receivers for receiving a desired RF signal on a specific RF channel (e.g., a desired bandwidth and a desired RF center frequency). One function of the RF receiver is to reject RF signals at frequencies other than those within the desired bandwidth of the desired RF channel. RF signals at bandwidths near the desired bandwidths are particularly troublesome.
One type of RF receiver is called super-heterodyning, in which received RF signals are mixed using mixers with a local oscillator (LO) signal to down convert the received RF signals into lower frequency signals, which are known as intermediate frequency (IF) signals. Generally, it is easier to filter out unwanted signals at IF frequencies than at higher RF frequencies.
Another type of RF receiver is called direct downconversion receiver. In this type of receiver, the received RF is mixed with LO signal at the carrier frequency of the received RF signal, and down converts the received signal directly to a baseband signal.
One type of down conversion using mixers is called quadrature down conversion. Quadrature down conversion produces two down-converted signals from the received RF signal. The down-converted signals are orthogonal, or 90 degrees out of phase, with each other. An in-phase mixer (or I mixer) mixes the received RF signal with a first or first set of local oscillator signals, producing a real down-converter signal (I signal). A quadrature mixer (or Q mixer) mixes the received RF signal with a second or second set of local oscillator signals, producing an imaginary down-converter signal (Q signal). The first and second local oscillator signals (or sets of local oscillator signals) are 90 degrees out of phase.
However, mixers have a characteristic that produces image signals in addition to desired signals. Image signals may be removed by RF filtering, IF filtering, or both. Bandpass filtering may be used to pass desired signals and remove interfering signals. Notch filtering may be used to target and reduce interfering signals at specific frequencies.
Another issue with RF receivers is sideband asymmetry in gains. The received RF signal within the desired bandwidth may experience different gains as frequency varies. For example, the received RF signal toward the upper side of the bandwidth is amplified differently than toward the lower side of the bandwidth.
The quality factor of a bandpass filter is a measure of the effectiveness of the filter at removing interfering signals at nearby frequencies, and is the bandwidth of the filter divided by the center frequency of the filter.
U.S. Pat. No. 8,121,577 introduced a filter embedded in the mixers. In that system, the output of a mixer is terminated with a polyphase reactive circuit, such as a capacitor. A mixer mixes RF input signals with LO signals and translates the impedance of the polyphase reactive circuit into the RF input impedance of the mixer. When fed from a high impedance source, such as a current source, the mixer provides a high quality factor (Q) impedance response associated with an impedance peak. The high-Q impedance response is used as a high-Q RF bandpass filter in a receive path upstream of down conversion, which improves receiver selectivity and replaces surface acoustic wave (SAW) or other RF filters.